Radiation Threat Detection

ABSTRACT

Systems and methods for detecting clandestine fissile or radioactive material on the basis of emitted radiation and particles (such as neutrons and alpha particles) arising from within the material. Emission by the fissile or radioactive material is detected in conjunction with a conventional x-ray imaging system that includes an external source of illuminating penetrating radiation, at least one detector configured to detect at least the penetrating radiation and to generate a detector signal, and a processor configured as a detector signal discriminator to generate an output indicating whether the detector signal is triggered by an origin other than illuminating penetrating radiation. Active and passive modes of detection are described by some embodiments. Other embodiments are directed toward neutron detection, gamma ray detection with energy resolution, and designs of detectors to enhance the detection of clandestine nuclear material.

The present application is a continuation application of U.S. patentapplication Ser. No. 13/650,709, entitled “Radiation Threat Detection”and filed on Oct. 12, 2012, which is a divisional application of U.S.patent application Ser. No. 12/239,054, filed Sep. 26, 2008, and issuedas U.S. Pat. No. 8,325,871 on Dec. 4, 2012, which is acontinuation-in-part application of U.S. patent application Ser. No.10/750,178, filed Dec. 31, 2003, which is in turn a continuation-in-partapplication of U.S. patent application Ser. No. 09/818,987, filed Mar.27, 2001, which claims priority from a U.S. Provisional Application withSer. No. 60/192,425, filed Mar. 28, 2000. U.S. patent application Ser.No. 12/239,054 is also a continuation-in-part application of a U.S.patent application Ser. No. 10/156,989, filed May 29, 2002, which claimspriority from a U.S. Provisional Application Ser. No. 60/360,854, filedMar. 1, 2002.

U.S. patent application Ser. No. 12/239,054, of which the presentapplication is a divisional application, is also a continuation-in-partapplication of U.S. patent application Ser. No. 10/620,322, filed Jul.15, 2003, which claims priority from U.S. Provisional Application60/396,034, filed Jul. 15, 2002, and is a continuation-in-part of Ser.No. 09/818,987, filed Mar. 27, 2001, which claims priority from a U.S.Provisional Application with Ser. No. 60/192,425, filed Mar. 28, 2000;and a continuation-in-part application of a U.S. patent application withSer. No. 10/156,989, filed May 29, 2002, which claims priority from aU.S. Provisional Application with Ser. No. 60/360,854, filed Mar. 1,2002.

Priority is claimed from all of the aforementioned applications, and allof the aforementioned applications are incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to methods and devices for inspectingcontainers, or objects contained therein, with penetrating radiation,while, during the course of the same inspection, searching for materialwith particular signatures (e.g., gamma rays, neutrons, electrons, oralpha particles) that provide an indication that the material might beeither fissile or radioactive.

BACKGROUND OF THE INVENTION

Sources of radiation and other nuclear material that might beclandestinely transported across national boundaries must be found. Thesources of radiation and clandestine nuclear material may be in the formof “dirty bombs” (e.g., a conventional explosive combined withradioactive nuclides designed to spread radioactive contamination upondetonation), fissile material, and other neutron and radiation emittingsources that may present a hazard to the public. During recent years,the United States government has placed mobile vehicles at strategicareas with gamma ray detectors dedicated to the task of finding fissilematerial. “Fissile material” includes those radioactive isotopesessential for nuclear explosives, and other isotopes found inconjunction with such radioactive isotopes.

Atomic explosives may be made from ²³⁵U, a rare, naturally occurring,isotope of uranium that lives almost 10⁹ years, or ²³⁹Pu, a reactor-madeisotope that lives more than 10⁴ years. ²³⁵U decays with the emission ofgamma ray photons (also referred to as ‘gammas’), principally at 185.6keV and 205.3 keV. ²³⁹Pu emits a number of gamma rays when it decays,the principal ones being at 375 keV and 413.7 keV. These gamma rays areunique signatures for the respective isotopes. But fissile materialinvariably contains other radioactive isotopes besides those essentialfor nuclear explosives. For example, weapons grade uranium may containas little as 20% ²³⁵U; the rest of the uranium consists of otherisotopes. The other uranium and plutonium isotopes reveal their presenceby gamma rays emitted by their daughters. For example, a daughter of²³⁸U emits a high energy gamma ray at 1,001 keV; a daughter of ²³²U, anisotope present in fissile material made in the former USSR, emits avery penetrating gamma ray at 2,614 keV; and a daughter of ²⁴¹Pu emitsgamma rays of 662.4 keV and 722.5 keV.

It may also be desirable to detect various other radioisotopes, orsignatures of such radioisotopes, that may be present in a “dirty bomb”.Detecting various isotopes of particular elements, such as cobalt orcesium, may be particularly valuable in attempting to deter terroristthreats.

U.S. Pat. No. 6,347,132, to Annis, describes seeking to detect nuclearweapons materials using an x-ray inspection system. However, Annisteaches that, in order to do so, one processes an x-ray transmissionsignal based on illuminating x-rays that traverse an inspected object,and, on the basis of characteristics (such as the spatial frequency offeatures, namely, how diffuse or compact they are) of the transmissionimage, in conjunction with the absence of scattering of the sameilluminating radiation from certain regions, one infers that nuclearmaterials might be present.

SUMMARY OF THE INVENTION

In one embodiment of the invention, an inspection system for inspectingan object includes an external source of penetrating radiation forgenerating an illuminating beam and for irradiating the object, at leastintermittently, the beam characterized at each instant of time by aninstantaneous energy spectrum and an intensity that may be substantiallyzero during particular (typically periodic) instants of time. Theinspection system also has at least one detector configured to detectpenetrating radiation including, but not limited to, penetratingradiation from the illuminating beam that is backscattered by theobject, and configured to generate a detector signal. Additionally, theinspection system has a processor configured as a detector signaldiscriminator to receive the detector signal, generate an x-ray imagebased on the detector signal that depicts at least illuminatingradiation backscattered by the object, and also to generate an outputthat may be perceived by an operator, indicating whether the detectorsignal is triggered at least in part by an origin other than thepenetrating radiation used to illuminate the object. The inspectionsystem has a display showing the x-ray image that depicts at leastilluminating radiation backscattered by the object, and has an outputthat indicates whether the detector signal is triggered at least in partby an origin other than the penetrating radiation used to illuminate theobject.

The detector signal discriminator may generate an output based on atleast one of source- and detected-signal timing and induced spectralcontent in the detector signal. The origin may include x-rays, beta raysfrom that result in the creation of x-rays, and neutrons. The detectormay include a segment having selective energy sensitivity. The detectormay also include two serial scintillators, one of which may be a heavyfluorescing material such as bismuth, gold, or lead. An x-ray absorbingwall may be interposed between the two serial scintillators. The sourceof penetrating radiation may be temporally gated by means such aselectronic gating or a chopper wheel that may include blocked spokes.The penetrating radiation may take the form of a pencil beam.

In a related embodiment of the invention, the system further includes acurrent-integrating circuit configured to receive the detector signal ofthe at least one detector; and a pulse-counting circuit configured toreceive the detector signal of the at least one detector, and to operateduring a period when the instantaneous energy intensity is substantiallyzero intermittently.

In another related embodiment of the invention, the at least onedetector includes a front scintillator and a back scintillator arrangedin series, the detected penetrating radiation traversing the frontscintillator before impinging upon the back scintillator, wherein thefront scintillator is more sensitive to the detected penetratingradiation below a given threshold than the back scintillator and theback scintillator is more sensitive to the detected penetratingradiation above the given threshold than the front scintillator. Thefront detector may be more sensitive to x-rays with energy below 100 keVand the back detector may be more sensitive to x-rays with energy above100 keV. The system may further include a converter configured toconvert energy of the detected penetrating radiation before the detectedpenetrating radiation is detected by the back detector. The convertermay be placed adjacent to a side of the back detector that is opposite aside facing the front detector.

Alternative related embodiments of the invention may alter theinstantaneous energy spectrum of the source to be capable of excitingcharacteristic emission lines of fissile elements, examples beinguranium and plutonium.

In another alternative related embodiment of the invention, the systemfurther includes a first scintillator capable of detecting neutrons andbeing less sensitive to gamma-rays and x-rays than neutrons; and asecond scintillator capable of detecting photons and being lesssensitive to neutrons than gamma rays and x-rays; wherein the detectionsignal discriminator generates an output when the origin includesneutrons from the object. The first scintillator may be a large areagadox screen, a ⁶Li-based scintillation screen, or a high pressure ³Heproportional counter. The second scintillator may be essentiallytransparent to neutrons, with the first and second scintillatorsserially arranged such that detected neutrons traverse the secondscintillator before impinging on the first scintillator. Alternatively,the second scintillator may be a moderator of fast neutrons and maycapture high energy photons, with the first and second scintillatorsserially arranged such that detected neutrons traverse the secondscintillator before impinging on the first scintillator. In such aninstance the second scintillator may be a plastic or liquidscintillator; the thickness of the scintillator may be in the range ofapproximately 2 cm. to 10 cm, and may be segmented.

In another embodiment of the invention, a directional detector ofradioactive emissions includes a scintillator for capturing an emissionfrom a radioactive source, the scintillator having a detection dimensionwith a total thickness greater than the mean free path of the emissionin the scintillator; and an optical detector configured to detectphotons emitted from the scintillator in a direction of the detectiondimension. The scintillator may emit photons after capturing neutrons,the neutron mean free path in the scintillator being shorter than thephotons mean free path in the scintillator. As well, the scintillatormay include at least two separate pieces separated by the opticaldetector, the optical detector being substantially neutron transparent.The directional detector may further include shielding configured toprevent the scintillator from capturing neutrons directed from adirection other than the direction of detection dimension. The shieldingmay substantially include at least one of ⁶Li, ¹⁰B, ¹¹³Cd, and ¹⁵⁷Gd.The directional detector may also include another optical detectorpositioned on an opposite side of the scintillator from the opticaldetector. The directional detector may further include a neutronmoderator material surrounding at least a portion of the scintillator.The neutron moderator may contain hydrogen, and may be made of highdensity polyethylene or water.

In an alternate embodiment of the invention, a method for detectingneutrons includes providing a scintillator containing highneutron-capture-cross-section atoms for capturing the neutrons andemitting electromagnetic radiation, at least one dimension of thescintillator exceeding the mean free path in the scintillator of aphoton of a specified wavelength range; and detecting photons at thespecified wavelength range with a photodetector characterized by aposition with respect to the scintillator. The method may include thestep of inferring a direction of a detected neutron with the position ofthe photodetector, and may further include the step of moderatingincident fast neutrons for capture by the containing highneutron-capture-cross-section atoms.

In another alternate embodiment of the invention, a method for detectingconcealed fissile material includes providing a first scintillatorscreen for absorbing massive fission products and generating visiblelight; a second scintillator screen in a path of photons that havetraversed the first scintillator screen; a heavy element backing in apath of photons that have traversed the second scintillator screen forgenerating Auger electrons and concomitant secondary photons; anddetecting visible photons arising in the first and second scintillators.

Another alternate embodiment of the invention is directed toward amethod for creating an x-ray image of an object and detectingclandestine nuclear material associated with the object. The methodincludes the steps of illuminating the object with penetratingradiation; detecting emission, including penetrating radiation,emanating from the object; producing an x-ray image of the object basedon the detected emission; and distinguishing between detected emissiondue to scattered penetrating radiation with the object and detectedemission due to the clandestine nuclear material. The step ofdistinguishing may include distinguishing detected emission due tofissile material or a dirty bomb. The detected emission may includegamma rays, x-rays generated by beta rays, or neutrons from theclandestine nuclear material. The step of illuminating the object mayalso include at least one of moving the object relative to a neutrondetector and moving the neutron detector relative to the object, andfurther include the step of correlating a detection signal from theneutron detector with the relative position of the neutron detector andthe object to identify the approximate location of a neutron emitter.The method may further include locating the clandestine nuclear materialassociated with the object using the x-ray image. The method may alsoinclude the steps of identifying a potential location of the clandestinenuclear material using the x-ray image; and redetecting emissionemanating from the object after repositioning the object based on theidentified potential location. The step of illuminating the object mayalso include illuminating the object intermittently, and the step ofdistinguishing may include distinguishing based on at least the source-and detected-signal timing. The step of distinguishing may also includedistinguishing based on at least a distribution of photon energies ofthe detected emission. The step of detecting emission may includedetecting emission due to x-ray fluorescence induced by interaction ofthe penetrating radiation with the clandestine nuclear material.

Another method associated with an embodiment of the invention isdirected towards creating an x-ray image of an object and detectingclandestine nuclear material associated with the object. The methodincludes illuminating the object with penetrating radiation; detectingemission, including penetrating radiation, emanating from the object;producing an x-ray image of the object based on the detected emission;and identifying heavy metal shielding (e.g., tungsten or lead) ofclandestine nuclear material associated with the object based on atleast identifying a characteristic emission line of active x-rayfluorescence produced by an interaction between the heavy metalshielding and the penetrating radiation. The heavy metal shielding mayalso be identified at least in part by a distribution of photon energiesof the detected emission.

In accordance with another aspect of the present invention, aninspection system is provided for inspecting an object. The system has abed moveable along a first direction having a horizontal component and asource of penetrating radiation coupled to move with the bed. Thesource, in turn, includes an intermittent beam transmitter of a kindblocking a penetrating radiation beam during a specified portion of eachperiod of recurrent periods of substantially constant duration, the beamcomprised of photons generated by the source of penetrating radiationfor irradiating the object. The system has a detector module coupledsuch that the detector module moves in coordination with the bed. Thedetector module has a scintillator sensitive to photons of thepenetrating radiation beam and a detector sensitive to particles havingmass that are emitted by fissile and radioactive material. The system,finally, has a radioactive-source-identifying detection circuitincluding an off-period discriminator particularly adapted, by virtue ofthe off-period discriminator, to distinguish particles detected by thedetector module during the specified portion of each period during whichthe penetrating radiation beam is blocked.

In alternate embodiments of the invention, the manifestation of thecapture of neutrons may be the emission of electromagnetic radiation. Asecond detector may be provided that includes an optical detector fordetecting the emitted electromagnetic radiation and generating anelectrical signal. The detector module may include a segment havingselective energy sensitivity with respect to photons of the penetratingradiation beam and electromagnetic radiation emitted by atoms of thedetector, and the detector module may also have at least one of acurrent-integrating circuit and a pulse counter.

The detector module may have two serial scintillators, and an x-rayabsorbing wall may be interposed between the two serial scintillators.

In accordance with yet another aspect of the invention, a method isprovided for inspecting a container. The method has steps of:

-   -   a. illuminating the container with a beam of penetrating        radiation comprised of photons from a source coupled to a mobile        platform;    -   b. detecting penetrating radiation emanating from the container;        and    -   c. distinguishing between detected radiation due to scatter of        photons from the beam of penetrating radiation and detected        radiation due to emission by fissile and radioactive material on        a basis of the detected radiation due to scatter of photons from        the beam being of lower energy per photon than the detected        radiation due to emission by fissile and radioactive material.    -   A further step may include activating an alarm upon detection of        emission by fissile and radioactive material.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of the invention will be more readily understoodby reference to the following detailed description taken with theaccompanying drawings:

FIG. 1 provides a top view of a cargo container being examined by twobackscatter x-rays systems, one on either side of the container, and twoorthogonal transmission systems, one horizontal, the other vertical, asan example of an inspection system that may be employed also fordetection of fissile and radioactive material in accordance with apreferred embodiment of the present invention;

FIG. 2A depicts a passive method for detecting ionizing radiationemitted from the target object, in accordance with an embodiment of theinvention, wherein, when no x-ray beam is present in the inspectiontunnel or when the x-ray beam is present and the counting rate is lowenough, the detector is switched to pulse mode;

FIG. 2B depicts a passive method for finding ionizing radiation emittedfrom the target object, in accordance with an embodiment of theinvention, wherein at least one of the spokes of the pencil beam formingwheel is filled with x-ray absorbing material so that no radiation fromthe x-ray source is sent into the target volume and the detector,switched to the pulse mode, is sensitive only to ambient radiation andradiation emitted from the target volume;

FIG. 3 depicts a passive method for detection of radioactivity in a modein which the detector is sensitive mainly to Compton x-rays, accordingto an embodiment of the invention;

FIG. 4 depicts an active method of radiation detection using a twochamber detector to measure the emissions from fissile material, inaccord with an embodiment of the invention;

FIG. 5 is a schematic view of an embodiment of the invention showing atwo-scintillation chamber detector configuration for detection offissile material in accordance with an embodiment of the presentinvention;

FIG. 6 is a schematic view of essential elements of a thermal neutrondetector in accordance with preferred embodiments of the presentinvention;

FIG. 7 is a schematic view of a 4π thermal neutron detector inaccordance with embodiments of the present invention;

FIG. 8 is a schematic view showing the use of backscatter detectors of acargo inspection system for detection of thermal neutrons in accordancewith preferred embodiments of the present invention;

FIG. 9 is a schematic view of essential elements of an enhanced x-ray orgamma ray detector in accordance with preferred embodiments of thepresent invention;

FIG. 10 is a schematic view of a combined enhanced photon detector andthermal neutron detector in accordance with further embodiments of thepresent invention.

FIG. 11 is a perspective view of a mobile cargo inspection systemdeployed on a truck capable of on-road travel to a location where thesame truck may be used to scan an enclosure such as a vehicle or cargocontainer in accordance with preferred embodiments of the presentinvention;

FIG. 12 is a perspective view of the device of FIG. 11 for inspectingvehicles or large containers for nuclear threats in accordance withembodiment of the invention; and

FIG. 13 is a side view of a further embodiment of a device forinspecting a large container in accordance with the invention;

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the invention are directed toward methods and devices fordetecting clandestine nuclear material such as sources of radiation,neutrons, and other particles (e.g., alpha particles). Whereas priorinspection systems are based on detecting radiation either transmittedthrough an inspected object or scattered by the object, the presentinvention additionally makes use of radiation that arises solely fromwithin the inspected object.

Some embodiments of the present invention are directed toward ways inwhich x-ray inspection systems, currently in use for detection ofcontraband materials such as drugs and conventional weapons andconventional explosives, may additionally be used for findingfissionable or radioactive material associated with scanned objects orin the containers they examine. Airport installations typically employlower energy (<250 keV) x-ray systems, while high energy (>450 keV)x-ray systems are becoming common at border crossings. Some techniquesare passive; i.e., the gamma rays from the clandestine nuclear materialsare the signatures for an alert. Several ways of carrying out suchpassive measurements are described.

Other methods, in accordance with the present invention, are active;i.e., the x-rays that illuminate a container excite fluorescence of amaterial and the characteristic emission is detected. These methods mayinvolve exciting the atoms of high atomic number materials such asuranium, plutonium, or lead, with a beam of x-rays, and looking for thefluorescence x-rays as the atoms return to their initial unexcitedground state. For example, a beam of x-rays with an end-point energy of225 keV can be used to excite uranium atoms, which then emit signaturefluorescence x-rays with energies of 94 keV to 111 keV. Photons that aredetected from either the passive or active modes may be the result ofBremsstrahlung radiation (i.e., photon emission from a material afterbeta particle interaction). Other embodiments of the invention aredirected toward designs of detectors that may be utilized to enhance theability to distinguish, or simultaneously, detect neutrons and radiationthat are indicative of the presence of fissile material or otherradiation sources such as dirty bombs.

Detection of Clandestine Nuclear Material

Some embodiments of the invention make use of systems in which a beam ofx-rays is swept through a plane of a container. X-rays transmittedthrough the container are detected in transmission detectors while x-raybackscattered from the container and its contents are detected in largearea backscatter detectors. In the discussion that follows, illustrativecalculations make use only of the backscatter detectors.

Inspection systems that may be used for practice of the presentinvention are of the variety described and shown in U.S. Pat. No.6,151,381, which is herein incorporated by reference. Other inspectionsystems that may be used for practice of the present invention are ofparticular utility for the inspection of large cargo containers such astrucks or sea/air containers in that they employ mobile platforms thatmay be driven past the inspected container during the course of theinspection. Such systems are described in U.S. Pat. No. 5,764,683, whichis incorporated herein by reference.

Referring now to FIG. 1, a top view is shown of a cargo container 10being examined by two backscatter x-ray systems 12 and 14, one on eitherside of container 10, and two orthogonal transmission systems, onehorizontal 16, the other vertical 18. These inspection systems are shownby way of example, and single inspection systems or differentcombinations of systems may be used within the scope of the presentinvention. One or more generators of penetrating radiation may be usedfor each of the transmission and scatter modalities.

Describing, first, backscatter x-ray systems 12 and 14, x-ray beam 20 isemitted by an x-ray source 22 of one of various sorts known to personsskilled in the art. Beam 20 may also be comprised of other forms ofpenetrating radiation and may be monoenergetic or multienergetic, or,additionally, of varying spectral characteristics. Backscatter x-raybeam 20 is typically generated by a DC voltage applied to the anode ofan x-ray tube 22 so that beam 20 is typically continuous. However, abeam 20 of other temporal characteristics is within the scope of theinvention. Beam 20 has a prescribed cross sectional profile, typicallythat of a flying spot or pencil beam and is scanned in time, as bychopper wheel 23, or another spatial modulator, thereby creating anoverall profile varying in time. The beam 20 may also have othergeometrical configurations, such as a fan beam. Beam 20 will be referredto in the present description, without limitation, as an x-ray beam.

Various means are known in the art for mechanically or electronicallysweeping a beam of penetrating radiation, including, for example, therotating chopper wheel 23 depicted in FIG. 1 or electronic scanning isdescribed in detail, for example, in U.S. Pat. No. 6,421,420, issuedJul. 16, 2002, which is incorporated herein by reference.

Penetrating radiation scattered by an object 27 within enclosure 10 isdetected by one or more x-ray detectors 26 and 28. X-ray detectors 28may be disposed at varying distances from x-ray beam 20 for differentialsensitivity to near-field objects 30 and far-field objects 27, asdescribed, for example, in U.S. Pat. No. 6,151,381. In order to obtaingreater spatial resolution of the source of scattered radiation,collimators 32 may be employed, as known to persons skilled in the x-rayart, for narrowing the field of view of segments of detector 28.

Transmission system 16 employs an x-ray beam 34 produced by source 36which is typically a high energy source of penetrating radiation such asa linear accelerator (Linac) for example. X-ray emission from a linearaccelerator is inherently pulsed, with typical pulse rates in the rangebetween 100 and 400 pulses per second. The portion of transmission beam34 which traverses enclosure 10 and objects 30 and 38 contained withinthe enclosure is detected by transmission detector 40.

The electrical output signals produced by detectors 26, 28, and 40 areprocessed by processor 42 to derive characteristics such as thegeometry, position, density, mass, and effective atomic number of thecontents from the scatter signals and transmission signals usingalgorithms known to persons skilled in the art of x-ray inspection. Inparticular, images of the contents of enclosure 10 may be produced by animage generator. As used in this description and in the appended claims,the term “image” refers to an ordered representation of detector signalscorresponding to spatial positions. For example, the image may be anarray of values within an electronic memory, or, alternatively, a visualimage may be formed on a display device 44 such as a video screen orprinter. The use of algorithms, as known in the art of x-ray inspection,for identifying suspect regions within the enclosure, and identificationof the presence of a specified condition by means of an alarm orotherwise, is within the scope of the present invention. When sospecified, an image uses the backscattered radiation as a key to thespatial distribution of the scattering material.

In many applications, it is desirable that enclosure 10 be inspected ina single pass of the enclosure through the x-ray inspection system.Enclosure 10 may move through the system in a direction indicated byarrow 46, either by means of self-propulsion or by any means ofmechanical conveyance of the enclosure with respect to the system.Alternatively, the enclosure 10 may not move while an arrangement ofdetectors and source of penetrating radiation may be rotated and/ortranslated with respect to the enclosure 10 to provide an x-ray scan ofthe enclosure 10. Detectors 26, 28, and 40, used in systems forinspection of the contents of baggage or cargo containers are typicallyoperated in a current integration mode rather than in a mode of countingindividual x-ray pulses by virtue of count rates that are typically toohigh to permit counting and processing individual x-ray pulses. Imagesof the distributions in the currents produced by the transmitted andbackscattered x-rays are typically built up as the container passesthrough the plane of x-rays.

Some embodiments of the invention may configure a processor to act as adetector signal discriminator in an x-ray system exemplified by featuresdepicted in FIG. 1; these x-ray systems may allow x-ray imaging systems,as known to those skilled in the art, to operate in conjunction with thedetector signal discriminator. The detector signal discriminator isconfigured to receive a detector signal from the system, and to generatean output indicating whether the detector signal is triggered at leastin part by an origin other than the penetrating radiation backscatteredby an object being scanned, or other than by any penetrating radiationused to illuminate the object. Processor 42 may operate as a detectorsignal discriminator, for example, by identifying the presence ofdetected signal during a portion of time when no illuminating radiationis incident on the object. FIG. 2B shows such an instant of time. Anysignal detected by detector 28 during the instant of time when theilluminating beam is blocked must be triggered by an origin other thanthe penetrating radiation used to illuminate the object.

Since beam 20 (shown in FIG. 4) is blocked by cowling 71 duringidentical portions of each revolution of chopper wheel 70, synchronousdetection of signal during the blocked intervals acts as aradioactive-source-identifying detection circuit including an off-perioddiscriminator particularly adapted, by virtue of the off-perioddiscriminator, to distinguish particles detected by the detector moduleduring the specified portion of each period during which the penetratingradiation beam is blocked.

The detector signal may be triggered by the detection of neutrons orpenetrating radiation (e.g., x-rays and gamma rays). The output may bebased at least on one of source- and detected-signal timing and inducedspectral content in the detector signal. Other embodiments of theinvention are directed to related and corresponding methods thatimplement a detector signal discriminator as described above. Thedetails of source- and detected-signal timing and induced spectralcontent in the detector signal are discussed herein in terms of passiveand active methods for discriminating radiation emanating from theinspected object as opposed to radiation derived from the illuminatingbeam, either directly or by scattering by atoms of the object.

Though some methods described herein refer to the specific detection ofuranium or plutonium, it is readily understood by those skilled in theart that the methods may be employed to detect other radioactivematerials. For example, ¹³⁷Cs, a commonly available radioactive isotopethat could be used by terrorists, emits a signature gamma ray at anenergy of 662 keV. Another common isotope is ⁶⁰Co that emits gamma raysof 1173 keV and 1332 keV. These gamma rays can be detected passively,may be indicative of the isotopes and the possible presence of a dirtybomb used by terrorists.

It is also to be understood that features of the invention need not berepresented in all figures depicting various embodiments; thus, forexample, while processor 42 is depicted with respect to the system shownin FIG. 1, it is not to be inferred that processor 42 may not equallywell be used in conjunction with components shown in all of the otherdrawings.

Passive Method I. Gated Detectors:

In some embodiments of the invention, discrimination of a detectorsignal is achieved by utilizing source- and detected-signal timing.Referring to FIGS. 2A and 2B, the x-ray beams 20 in x-ray inspectionsystems typically sweep, as by rotation of chopper wheel 70, through theinspection volume during a large fraction of the operating time. Duringthe remaining fraction of each sweep cycle there are essentially nosource x-rays striking the target container. Thus, during the time ofsource quiescence, the detectors are only counting background.Alternatively, one or more apertures of the chopper wheel 92, 94 may beremoved or blocked, as shown in FIG. 2B, by a material that is opaque toa range of x-ray energies, thus increasing the period in which detectorsmay be counting photons while an object is not illuminated by an x-raybeam. Such a material constitutes the cross-hatched cowling 71 shown inFIGS. 2A, 2B, 3, and 4. Thus an increase in sensitivity and efficiencymay be achieved while searching the target volume. Other mechanicaldevices may be substituted to create x-ray beams in a manner thatprovides a time of source quiescence.

In another alternative, a beam that is produced by an x-ray system maybe gated electronically (i.e. turned on and off electronically) to allowbackground measurements. Electronic gating of the pencil beam, forexample by the use of a gating grid in the x-ray generator, is apreferred method that gives flexibility to the procedure and obviatesthe need to add shielding material. This alternative may have advantagesover mechanically blocking the beam that include having less x-rayleakage during the source quiescence time, and faster on/off times.

In the geometry of FIG. 2A, a beam 20 is directed into the target areafor about 95% of the time. Thus, a target container is being inspectedby the x-ray beam 20 for 95% of the time it is in the target chamber,leaving only 5% of the time for counting radioactivity without a beamstriking the container. The “quiescent-time” is typically only 0.2seconds (5% of 4 seconds of inspection per bag) but may still be capableof finding clandestine radioactivity as the following example shows.

The 185.6 keV gamma rays are emitted in 53% of the decays of ²³⁵U (shownas object 76) but only a thin layer of the bulk uranium is accessiblesince the mean free path of 185.6 keV gammas in uranium is only 0.36 mm.Still, every square centimeter of 10% enriched uranium will emit ˜twothousand 185.6 keV gamma photons per second, giving rise to a count of2,000×0.004=8 counts for every square centimeter of surface area ofuranium that faces the detectors. A 1″ cube of uranium (weighing ˜¾pounds) would signal its presence with ˜50 counts in the 0.2 secondoff-period of the inspection. A signal of this magnitude is easilydiscriminated.

The signal from clandestine radioactivity relative to the backgroundnoise can be enhanced substantially in a number of ways includingincreasing the off-time of the sweeping x-ray beam as shown in FIG. 2B,improving the detection efficiency, increasing the solid angle ofdetection, and lowering the noise level at the gamma ray energies ofinterest.

In a preferred embodiment, particularly useful for lower energy (140keV-200 keV) x-ray systems, the noise level at the gamma ray energies ofinterest can be substantially reduced by switching the output frombackscatter detectors 28 to a pulse counting circuit 72 during thefraction of the operating cycle during which the source of x-rayirradiation is off. During this period, individual 185.6 keV gamma rays74 can be detected and analyzed with low noise levels at that energy.Pulse counting circuit 72 may be used in conjunction with otherembodiments of the invention other than that depicted in FIG. 2B, andconstitutes a particularized feature of the detector circuitry thatallows processor 42 (shown in FIG. 1) to discriminate between detectionsmade when the source of x-ray irradiation is not providing anilluminating beam incident on the inspected object.

In another preferred embodiment, the pulse-counting mode is utilizedwhenever the count rate in the backscatter detectors falls below apredetermined value, for example 100,000 counts/sec, whether or not thex-ray beam is being sent into the target chamber. The predeterminedmaximum count rate is chosen as that rate at which it is still practicalto measure the energy of the individual photons. When the energy ofindividual photons detected in backscatter detectors can be analyzedthen it becomes practical to search for the 185.6 keV gamma rays from²³⁵U even while imaging the luggage. The reason is that the energies ofthe Compton backscatter x-rays, produced by the incident x-ray beam 20,are always lower in energy that 185.6 keV and therefore do notinterfere. Specifically, the maximum backscattered Compton energies forx-ray beams produced by electron beams of 160 keV, 220 keV and 440 keV(the maximum energy used in any commercial backscatter system) are 104keV, 127 keV and 178 keV. A luggage security system such as shown inFIG. 1 may, therefore, advantageously continue to operate normally whilethe fissile detection system operates efficiently in the background, asdescribed.

In another preferred embodiment, one or more of the spokes are solidrather than hollow. FIG. 2B shows two, diametrically opposite spokes 92,94 that are blocked. In that example, the beam is restricted from thetarget chamber for slightly more than a third of the time, or about 1.3seconds. The backscatter detectors operate for about 1.3 seconds asdetectors of radioactive material in a piece of luggage, free fromradiation correlated with the x-ray beam 20; a gain in sensitivity ofmore than a factor of six.

In still another preferred embodiment, a backscatter detectorconfiguration is proposed that operates in the current mode, as opposedto the pulse counting mode, and specifically looks for the 185.6 keVradiation while luggage is being examined with the radiations from thex-ray beam.

Passive Method II. Continuous Detection:

Several modes are described to search, during the imaging time, for theradioactive emission of the 185.6 keV gamma ray from ²³⁵U, and otheremissions in the range from approximately 100 keV to 200 keV. Some modesutilize the fact that the maximum energy of the Compton backscatteredx-rays that form the x-ray backscattered image is less than the soughtfor 185.6 keV gamma ray.

The minimum energy of the gamma rays from fissile material is 187 keV.The maximum energy of x-rays detected in the backscatter counters isgiven by:

${E^{scattered} = \frac{E^{incident}}{\left( {1 + \frac{E^{incident}\left( {1 - {\cos \; \theta}} \right)}{m_{e}c^{2}}} \right)}},$

where E^(incident) is the energy of an incident photon, E_(scattered) isthe maximum energy of a scattered photon, m_(e)c² is the rest energy ofan electron, and 2 is the scattering angle. In the backward direction,E^(scattered) is typically only 100 keV for 160 keV x-ray generators and170 keV for 450 keV generators. The preponderance of detected x-rays,thus, in either passive or active inspection modality, have energieswell below 100 keV. It is therefore feasible to count continuously (thatis, during the inspection itself) with a detector that has a thresholdat say 160 keV, a straightforward task if the radiation is detected witha pulse counter.

As shown in FIG. 3, the radiation detectors of certain x-ray inspectionsystems, measure current; i.e. the charge integrated over specifictimes, as measured by a current-integrating circuit 80. A current modecounter 80 sensitive to the fission material gamma rays may beimplemented by the following method using a segmented detector havingselective energy sensitivity.

Referring now to FIG. 4, a two-chamber backscatter detector is used. Afront chamber 50, through which the fluorescing and fissile materialradiation 52 and 54 pass through first, has very good efficiency fordetecting the radiations below about 100 keV; i.e., the bulk of theCompton scattered radiation. A rear chamber 56, with thicker, higher-Zscintillators, has very good efficiency for detecting radiation up to200 keV. An opaque wall 58 between the front and rear chambers may be anabsorber properly chosen to further reduce the lower energy radiationswhile passing the higher energies that signify the presence of gammarays emitted from container 10.

The ratio of the current pulse in back detector 60 to that in frontdetector 62, as depicted in FIG. 5, is a good measure of the presence orabsence of higher energy gamma rays. When there is no radioactive sourcein the inspected container, the ratio will have a range of values thatwill always be lower than the range of values when the gamma rays fromuranium and plutonium are present.

In using the passive modes of detection, an alarm may be triggered bythe system upon suspected detection of clandestine nuclear material. Toverify the reality of an alarm, as determined by a high ratio, the x-raybeam may be switched off while the container is still in the inspectionvolume. This passive detection modality allows for a more carefulmeasurement of the passive radiation emissions.

Active Detection:

Photoelectric interaction takes place in uranium and plutonium when theelements are bombarded with x-rays greater than 115.6 keV and 121.72keV, respectively. The excited atoms then decay back to their groundstates by isotropically emitting their characteristic K_(α) and K_(β)x-rays. The energies of the K_(α) x-rays of U are 94.6 keV and 98.4 keV,while those of Pu are 99.4 keV and 103.7 keV. The energies of the K_(β)x-rays of U are 111.3 keV and 114.5 keV, while those of Pu are 117.1 keVand 114.6 keV. These are characteristic x-rays with a uniquely highenergy; the characteristic K x-rays of lead, the heaviest element thatmight be found in any quantity in a container, span the much lower rangefrom 72.8 keV to 87 keV.

The x-ray generators used for inspecting luggage and smaller containershave maximum energies of 140 keV to 160 keV. The components of the x-rayspectrum above 115.6 keV and 121.72 keV (the K-electron binding energiesof uranium and plutonium respectively) interact with the fissileelements through the photoelectric effect. The result is that the entireenergy spectrum above the binding energies is effectively converted intothe characteristic x-rays of the elements.

High-energy x-rays are readily detected with detectors operating in thepulse-counting mode, as known in the art. When the detectors areoperated in a current integrating mode, it is necessary to useunorthodox methods.

The use of the simple two-chamber method described above in the contextof passive measurements is typically not preferred here because thecharacteristic x-ray emission of the fissile material is at considerablylower energy than the gamma rays emitted by the fissile materials.

In accordance with a preferred embodiment, the two-scintillation chambermethod described above is modified in the manner now discussed.Referring further to FIG. 4, front chamber 50 is, as before, especiallysensitive to energies below 100 keV, while back chamber 56 isspecifically sensitive to energies up to 130 keV. Cooled germaniumdetectors are preferably utilized, though other types of detectors mayalso be employed with such embodiments of the invention. The unusualfeature of the back scintillation chamber, described now in reference toFIG. 5, is the inclusion of a layer of bismuth 58, approximately a meanfree path thick, for energies above its K edge. X-rays below the Kbinding energy see the bismuth as a relatively thin absorber and thephotoelectrically induced L x-rays have too low an energy to beeffectively detected. Alternatively, other heavy metal materials such aslead or gold may be used in place of bismuth. X-rays above 90.5 keV,however, are converted into x-rays of 74.8 keV and 89.8 keV, which aredetected in scintillation detectors of the back detector. The conversionefficiency depends on the energy of the x-ray or gamma ray beingconverted; it is ˜50% for 185.6 keV. When no fissile signal is present,the ratio of the current integrated signals in the back detector to thatof the front detector is lower than the ratio when fissile material ispresent. Thus the ratio can be used to automatically alarm on thepresence of fissile material. Sensitivity to 100 gram amounts of fissilematerial is readily achieved.

A simple calculation, referring to FIG. 5, shows the effectiveness of atwo-chamber method of detection. Ray 52 represents backscatteredradiation from a container 10 that has negligible fissionable materialor any very heavy element such as lead or gold. The spectrum ofbackscatter 52, generated from an x-ray generator with a maximumelectron energy of 160 keV, has few x-rays above 100 keV. Typically, forevery 100 x-ray photons with energy in the range of 60 keV to 75 keV,there will be less than 5 x-ray photons above 100 keV. Ray 54 representsbackscatter and additionally K x-rays fluoresced from fissile materialin container 10.

Front chamber 50 has a scintillator 62 of, for example, 200 mg/cm² ofGdOS, which has an efficiency of ˜70% for counting the 60 keV to 75 keVradiation, but only a 30% efficiency for counting the x-rays above 100keV. Thus, the signal in chamber 50 will consist of 70 counts from the60 keV to 75 keV x-rays and 1.5 counts from the x-rays above 100 keV.Passing out of chamber 50 into chamber 56 are 30 x-rays in the 60-75 keVrange and 3.5 x-rays above 100 keV.

The x-rays that enter chamber 56 pass through a bismuth absorber 58which has a 70% efficiency for stopping the x-rays in the 100 keV to 120keV range and a 50% efficiency for stopping the x-rays in the 60 keV to75 keV range. Thus 15 of the 30 x-rays of 60-75 keV stop in the bismuth;the produced L x-rays have too low an energy to be counted in thescintillator 60. Scintillator 60 is similar to scintillator 62 and ittherefore counts ˜10 x-rays of 60-75 keV and less than 1 x-ray greaterthan 100 keV. The ratio of counts in chamber 56 to chamber 50 will be10/70=˜0.14; the ratio of currents will be almost the same.

The case when fissile material is present is now considered, withshielding around the fissile material neglected for clarity. A 140 keVx-ray beam produces approximately 500 characteristic fluorescence x-rays54 from every square centimeter of uranium or plutonium that is struckby the beam. Approximately 100 of those fluorescence x-rays will enterchamber 50 and 30 of them will stop and be counted. The total counts inchamber 50 will then be 70+1.5+30=101.5.

The 70 fissile-induced x-rays 52 that penetrate into chamber 56 willinteract with the bismuth and 50 of them will stop and produce bismuthx-rays 64 of 75 keV to 100 keV. These will be counted in chamber 56 withan efficiency of ˜70% so that the bottom chamber will count ˜35 x-raysover and above the count were fissile material to be absent. The ratioof counts, or current, between chamber 56 and 50 will rise from ˜0.14 toalmost 0.5, a readily distinguished change. Use of a pencil beam forx-ray irradiation of the inspected enclosure allows determination of theoutline and position of the fissile material, using standard x-rayinspection algorithms. Upon detection of fissile material, processor 42(shown in FIG. 1) may then give rise to activation of an alarm 43, and,additionally, display the outlines of the fissile material on display 44using highlighting coloring or other standard techniques.

It is also to be noted that some of the active modes described hereinmay equally be used to detect high atomic number shielding materials,such as lead or tungsten, enabling the detection of clandestine shieldedradioactive sources. For example, lead shielding, which may be used toconceal radioactive sources, may be detected using these methods sincethe lead atoms emit signature fluorescence x-rays at 72.8 keV to 84 keVwhen excited by x-rays with an end-point energy of 225 keV. Similarly,tungsten emits fluorescence at 59.7 keV to 67 keV when excited by anx-ray beam with end point energy of 225 keV. Thus, heavy metal shieldingmay be identified based on identifying the specific energy of activex-ray fluorescence expected from an interaction between the heavy metalshielding and the exciting x-rays. Alternatively, the excitation of theheavy metal shielding may result is a distribution of fluorescencephoton energies, which may be used to identify the presence of theshielding.

It is to be noted, also, that the passive and active modes describedherein may advantageously also be employed in conjunction with x-rayinspection systems employing a fan beam, or otherwise shaped beams, suchas standard transmission-imaging systems commonly employed for luggagescrutiny at airports. The preferred position, however, for the fissiondetectors is in the back direction, i.e., on the same side, with respectto the inspected object, as the x-ray generator. In that geometry, theenergy of the x-rays Compton-scattered from material in the containerwill be lowest and furthest in energy from the high-energycharacteristic x-rays, or gamma rays, emanating from the fissionablematerial.

In other embodiments of the invention, the passive and active modesdescribed herein may be enhanced through the use of conventional x-rayimaging capabilities of an inspection system. For example, the activeand passive modes may include the use of an alarm (e.g., an optical oraudio signal) to indicate that a detection signal is triggered at leastin part by the presence of clandestine nuclear material. Upon thetriggering of an alarm, the x-ray image corresponding to the scannedobject may be examined to determine the location, shape, or othercharacteristics of the clandestine nuclear material. As well, the x-rayimage may be used to reposition the scanned object or container toincrease the efficiency and accuracy of detecting clandestine nuclearmaterial. In the passive modes, this may include positioning detectorsto detect emissions from to suspected clandestine material. In theactive modes, this may include positioning a scanning x-ray beam toexcite the suspected clandestine nuclear material and positioning adetector to detect the fluorescence.

Designs for Neutron and Radiation Detectors

Embodiments of the invention discussed herein may be utilized to improvethe detection of clandestine nuclear sources. In particular, someembodiments may be utilized in conjunction with the modes describedearlier to detect clandestine nuclear material.

Some embodiments of the current invention are directed toward thedetection of thermal neutrons. Thermal neutrons fluxes above the lowambient background are produced by specific radioactive sources,so-called AmBe and PuBe sources, or by the spontaneously fissioningisotope ²⁴⁴Cf, or by sources of plutonium; the detection of the latterbeing prima facia evidence for atomic bomb material. Commercial sourcesof thermal neutrons are rarely found outside fixed installations so thatthe presence of thermal neutrons above the ambient levels is cause foralarm. Thermal neutrons have traditionally been detected by commerciallyavailable ³He or BF₃ counters or by plastic or glass scintillators dopedwith ⁶Li or ¹⁰B.

In embodiments of this invention, the neutrons are detected in speciallarge area scintillator screens that have a higher efficiency fordetecting thermal neutrons than detecting x-rays or gamma rays. Suchembodiments may be used in connection with a conventional x-ray imagingsystem (e.g., a system as depicted in FIG. 1 and described earlier) tosearch for clandestine nuclear material that includes a neutron emitter.A preferred embodiment makes use of material that is more sensitive tothermal neutrons than backscattered x-rays, with a detection ratiogreater than approximately 10⁶. An example of such material is a 20mg/cm² gadox scintillation screen. Gadox (Gd₂O₂S) has a high efficiencyfor detecting thermal neutrons, but a low detection efficiency fordetecting x-rays; it is almost transparent to photons above about 100keV. The different mean free paths between thermal neutrons andparticular energies of x-rays can be used to make a “4π” neutron onlydetector, or a directional neutron detector, or a “4π” neutron plusx-ray detector, or a directional detector for both x-rays and neutrons,as discussed in greater detail below. Though some embodiments of theinvention described herein refer to the specific use of gadox as aneutron and radiation scintillator, it is readily understood by thoseskilled in the art that the embodiments are not limited to the use ofgadox, and may be practiced with any material with appropriate neutronor radiation interaction properties adjusting for the appropriate meanfree path of neutrons and photons. Of particular interest are the⁶Li-based scintillation screens (e.g., LiF) which have good efficiencyfor detecting thermal neutrons, i.e. 30% or greater, while beingvirtually invisible to x-ray or gamma ray radiation. Another neutrondetector of interest is a high pressure ³He proportional counter.

The detection of gamma radiation may be accomplished using detectorsthat are also used to create the backscatter and transmission x-rayimages. The backscatter detectors, which typically consist of a bariumbased scintillator (e.g., BaFCl₂ phosphor screen), are most efficientfor detecting the lower energy gamma rays, in the energy range belowabout 100 keV. The transmission detector, which consists of plasticscintillator, is most efficient for detecting the higher energy gammarays above about 150 keV.

In some embodiments of the invention, a high-energy detector may beadded behind or beside the x-ray backscatter detectors used to form abackscatter image of the x-rays 20. This addition is especiallyimportant when embodiments of the invention do not utilize atransmission imaging detector. The high-energy system may be one of manytypes of commercially available gamma detectors, including NaI(Tl), BGO,CsI(Tl). In a preferred embodiment, the gamma ray detector is asegmented, large area plastic scintillator that is very well shieldedfrom gamma radiations of energies below 200 keV; a segmented large arealiquid scintillator may also be appropriate in some applications.

The plastic or liquid scintillator approach is attractive for severalreasons. The cost per unit area for plastic or liquid scintillators isby far the lowest for detectors of the same efficiency. Plastic orliquid scintillators have very poor energy resolution and will not beable to identify the radioactive element that emits the high-energyphotons it detectors. But they will serve the purpose of quickly andefficiently finding, hidden high-energy gamma ray sources such as ¹³⁷Csor ⁶⁰Co. Identifying the isotope will be the task of an auxiliarydetector that may be a hand-held probe with good resolution that canidentify the emitting isotope. The segmentation of the plastic or liquidscintillator is preferably along the direction of travel of thecontainer so that, knowing the speed of the container moving by thedetector it is straightforward to determine the approximate origin ofthe radioactive source in the container.

The plastic or liquid scintillator has the further important advantagethat it can serve the dual purpose of an efficient gamma ray detectorand an efficient moderator of fast neutrons. A preferred embodiment usesa segmented plastic or liquid scintillator with a total area that isequal to, or greater than, the area of the neutron detector and placedjust in front of the neutron detector, that is, on the side facing thetarget volume.

In some embodiments of the invention, a neutron detector may be operatedsimultaneously with an x-ray imaging inspection system. The followingdescription uses the scintillator gadox as the example of an efficientneutron detector. It should be understood that other neutron detectors,known in the art, could serve as well or better. For example, ⁶Li isincorporated in some scintillation screens to give good neutrondetection efficiency. The large energy released when ⁶Li captures aneutron can be used to discriminate neutron from photon interactions,making the screens invisible to gamma radiation. The methods of making a⁶Li detector sensitive to the neutron direction would be similar tothose described below for gadox.

FIG. 6 shows the essential elements of a neutron detector 600. A gadoxscintillator screen 100, covered by an optical shield 800 that reflectsthe internally generated light, is viewed by a PMT 200. Thermal neutronsentering the gadox 100 along path 120 are absorbed by the ¹⁵⁷Gd,producing Auger (internal conversion) electrons 140 with energies in the24 to 70 keV range. These electrons 140 stop in the gadox 100 producingoptical photons 160. The optical photons 160 are captured by thephotocathode 180 of the PMT 200, producing a signal at the anode that isprocessed by pulse electronics 210. The neutrons 120 are stopped in thefirst 20 mg/cm² of the gadox 100. The optical photons 160 have a maximumtravel of about 200 mg/cm² in gadox. A region 400 of a moderatormaterial, such as paraffin, for example, may be provided in order toslow any fast neutrons 230 and enable their detection in the mannerherein described with respect to thermal neutrons.

If the gadox is thicker than the maximum optical photon travel distance,then neutrons stopping in the outer layer are not detected. Neutronsentering the gadox from the side facing the PMT photocathode are readilydetected.

Referring now to FIG. 7, an embodiment of the invention in the form of a“4π” neutron-only detector is made by placing a 20 mg/cm² thick gadoxscreen 100, with its accompanying optical shield and reflector 800 andphotodetector(s) 200, in a box 220 shielded by a material 240 opaque tox-rays. Shield 240 may be 5 mm thickness of bismuth or 1 cm of lead, tocite two examples. Bismuth, with a mean free path of 17 cm for thermalneutrons, is essentially transparent to the neutrons but effectivelyshields the gadox from x-rays and gamma rays that have any appreciableinteraction with the gadox.

To make a directional detector of neutrons, the gadox need only be muchthicker than the mean free path of the optical light, say 300 mg/cm².The light detected by a PMT must have come from neutrons that enteredfrom the side of the gadox facing the photocathode of the PMT. Thisembodiment, when unshielded with respect to x-rays, is also adirectional detector for x-rays that have a mean free path much lessthan 150 mg/cm² in gadox. And when this detector is placed in a properlyshielded box, it may be rendered sensitive to neutrons only.

Other materials that may be used to shield neutrons include ⁶Li, ¹⁰B,¹¹³Cd, and ¹⁵⁷Gd.

In another embodiment of the invention, neutron detection may beenhanced by combining a neutron detector with an x-ray scanning systemto aid identification of the location of a neutron source associatedwith a scanned object or container. An x-ray scanning system maytranslate an object relative to a neutron detector. Thus, the counts ofa neutron detector and the position of a scanned object or container maybe correlated to identify the location of a neutron emitter associatedwith an object or container. If imaging is part of the x-ray scanningsystem, the generated image may also be correlated with the counts ofthe neutron detector to improve the ability to locate the neutronemitter. Other more precise detectors may also be subsequently utilizedto further characterize or confirm the detection of a neutron emitter.

Alternative versions of the invention for detection of neutrons andx-rays make use of back-to-back gadox screens separated by an opaquefilm, or combinations of gadox and scintillation screens that do notcontain gadolinium and are essentially transparent to neutrons.

In accordance with further versions of the invention, a moderator, suchas paraffin, is employed to convert fast neutrons into thermal neutrons.Thus, fast neutrons, such as those emitted by plutonium, mayadvantageously be detected in the manner described above with respect tothermal neutrons. Examples of other moderator materials includematerials containing hydrogen, including high density polyethylene andwater.

Another embodiment of the invention utilizes a moderator that may alsoact as a high energy photon detection screen. Plastic or liquidscintillators may be used to capture high energy photons (e.g. photonswith energies above 200 keV), while also slowing fast neutrons. Thethickness may be tailored to the particular application; for some of theexamples discussed herein, plastic or liquid scintillators withthicknesses in the range of approximately 2 cm. to 10 cm strike thecorrect balance of moderating neutrons and allowing lower energy photonsto pass through. Such a moderator may also be segmented. The moderatoris typically arranged serially with a neutron scintillator to moderateneutron velocity before the neutrons impinge upon the neutronscintillator.

One preferred embodiment of the gadox detectors is in x-ray inspectionsystems to find neutron-emitting material in baggage at airports or infreight cargo. Referring now to FIG. 8, baggage or cargo 300 isirradiated by x-ray beam 310, typically swept as a pencil beam in ascanning pattern across object 300. Other beam shapes, however, arewithin the scope of the present invention. Detectors 312 ofbackscattered x-rays 314 are hollow rectangular boxes whose innersurfaces are lined with gadox 316, approximately 150 mg/cm² thick, or,alternatively, gadox and another scintillator that does not absorbneutrons. Photomultipliers 200 intrude into the boxes to detect thefluorescent light. In order to transform the backscatter detectors intoefficient neutron detectors as well as x-ray detectors, the gadox 318 onthe surfaces facing the inspected container 300 are thin enough thatfluorescent light from the neutrons stopped in the outer 10 mg/cm² layercan efficiently escape out of the scintillator and be detected by thePMTs. A more effective solution is to add a separate section to thebackscatter detectors with additional PMTs.

Alternatively, thick gadox can be placed on the surface furthest awayfrom the inspected container, and a scintillator that is essentiallytransparent to neutrons can be placed on the surface facing theinspected container. In this embodiment, the neutrons will be absorbedon the inner surface of the gadox, allowing the scintillation light tobe detected by the PMTs. Gamma rays and x-rays will be absorbed anddetected in both the gadox and the other scintillator. In this way, theefficiency for absorbing and detecting high energy x-rays or gamma raysmay be maximized.

Gadox scintillation screens can be placed, in accordance with otherembodiments of the invention, on traditional gamma ray detectors thathave excellent efficiency for detection of both high and low energyx-rays or gamma rays. In this embodiment, the gamma ray detectors act aslight conduits for the fluorescent light produced by the gadox screens.For example, the gadox can be optically coupled to plastic scintillatorsviewed by PMTs to efficiently detect high energy gamma rays.Alternatively, the gadox can be optically coupled to high-Z gamma raydetectors such as NaI(Tl), BGO, CsI(Tl), etc. The signals from the twodistinct scintillators, one of which is gadox, can generally be viewedby a single PMT with the signals from the two scintillatorsdistinguished by their different pulse decay times, a technique wellknown in the art. When two distinct scintillators, one of which isgadox, are viewed by two PMTs, the signals from the two scintillatorscan be separated by placing a notch filter for the 511 nanometer line onone of the PMTs so that it only counts light from the gadox.

Gamma Ray Detection Enhancement

In accordance with further embodiments of the invention, the detectionefficiency of scintillation screen detectors is enhanced for x-raysabove about 70 keV, with particular utility for x-ray energies in the100 keV to 200 keV range.

Advantage is taken of the fact that heavy materials such as tungsten,lead and uranium are excellent converters of higher energy photons tolower energy photons, which, in turn, are more efficiently detected bythe gadox. The invention is now described with reference to theschematic shown in FIG. 9.

X-rays 420, 440, 480 or gamma rays 420, 440, 480 impinge on the detector410, which consists of a scintillator 370, such as gadox, lining theinside of the front face, a scintillator 320, such as gadox, lining theinside of the back face of the detector, PMTs 360 viewing the interiorof the detector to measure the intensity of the light emitted from thegadox, and a sheet 340 of a heavy element, such as lead, backing thescintillator 320.

The operation of the detector is illustrated by imagining that 100, 100keV x-rays (such as the K x-ray of uranium) and 100, 185.6 keV gammarays (from the decay of fissionable ²³⁵U) impinge on the detector. Thescreens 370, 320 are assumed to be 150 mg/cm² gadox, their maximumeffective thickness. The backing 340 is assumed to be 5 mm of lead,which is thick enough to stop the 100 keV and 185.6 keV photons. Weconsider each radiation in turn; the numbers in the examples areapproximations provided solely for purposes of illustrating theprinciples described herein.

The front gadox layer 370 stops and detects about 30 of the 100 keVx-rays, letting 70 x-rays through. The back gadox layer 320 stops 21 ofthe 70 x-rays so that if the lead sheet 340 were not present, 49 x-rayswould pass out the back end of the detector; the efficiency of thedetector for 100 keV x-rays would be ˜50%.

In the configuration of FIG. 9, the remaining 100 keV x-rays 480 stop inthe lead sheet, 340. The stopping is primarily the result of the 100 keVx-rays ejecting K electrons 500 from the lead atoms (the photoelectriceffect). When the excited lead atoms deexcite, which they do in lessthan a picosecond, K x-rays are emitted with energies of 72 and 75 keV;for illustration we use the dominant 75 keV x-ray. The fraction ofincident x-rays that result in K x-rays emitted backwards into thedetector is given by the ratio,

F˜μ(photoelectric)/[μ(total for 100 keV)+μ(total for 75 keV)]=0.68.

The actual fraction will be lower because of the finite angular spreadsand the finite fluorescent yield, but will still be close to 50%. Thus,about 25 of the 49 x-rays that entered the lead backing will result in72-75 keV x-rays reentering the chamber. The 300 mg/cm² of gadoxcaptures 80% of these x-rays so that ˜20 of the 49 x-rays are detected.The lead backing has increased the efficiency of the detector for 100keV x-rays from 50% to 70% at a cost of a sheet of lead.

The calculation for the 185.6 keV gamma ray impinging on the detectorproceeds similarly. Of the 100 incident gammas, only 9% interact at allin the gadox, so that more than 90 gamma rays penetrate into the lead.Approximately 30% of these gamma rays produce lead K x-rays that reenterthe backscatter detector. The result is that approximately 20 additionalgamma rays are detected over what would have been detected without thelead. The efficiency of the detector has increased from 9% to 29% by theaddition of a sheet of lead.

Combined Neutron Detector and Enhanced Gamma Ray Detector

The invention described herein is advantageously applied to theefficient detection of fissionable material that may be transportedillegally by smugglers or terrorists. FIG. 10 shows a simple detector800 that has excellent efficiency for thermal neutron capture, and goodefficiency for both the K x-rays of uranium or plutonium and the 185.6keV gamma rays emitted by fissionable uranium. Detection of fastneutrons by employing an intervening moderator is discussed elsewhereherein.

The configuration is similar to that of FIG. 9, the only substantivedifference is that the first gadox layer 510 is only 20 mg/cm² thick.The front layer stops the thermal neutrons 520 producing strong signalsin the PMTs 700 corresponding to the deposition of 25 keV and 70 keVelectrons. A fraction of the 100 keV and 185.6 keV photons 540 stop in aback scintillator layer 600, the remainder stop in the lead backing 620.The back scintillator layer 600, in accordance with one embodiment, mayitself be gadox or another high-neutron-capture material, and, morespecifically, may have a thickness of 150 mg/cm².

The overall detection efficiency for the 100 keV and 185.6 keVradiations is about 60% and 25% respectively. The detection efficiencyfor thermal neutrons is ˜50%.

Other configurations of successive scintillator panels are also withinthe scope of the present invention. If the first detector is thin gadoxthen the second detector can be any scintillator with good stoppingpower, including gadox. Alternatively, the first detector may be ascintillator other than gadox while the second detector is gadox.

If the second detector is gadox, then, within the scope of the presentinvention, the first detector can be thin gadox or a scintillator otherthan gadox.

In another embodiment of the invention, the front layer 510 is a neutronscintillating material with a low efficiency for capturing gamma-rays orx-rays below a given threshold energy. The second layer 600 is ascintillator with a higher efficiency for capturing gamma-rays or x-raysabove the given threshold, while having poor to zero efficiency forcapturing neutrons. The capture of photons 560 by the second layer 600may be enhanced by employing a heavy metal backing 620 that is chosen toproduce Auger electrons 640 that are subsequently absorbed by the secondlayer 600, in a manner similar to what is described earlier.

Some embodiments of the invention make use of systems in which detectorsare deployed at a fixed site, as may be appropriate for the screening ofparcels or baggage, such as mail shipments or luggage carried bypassengers. Other preferred embodiments of this invention make use ofsystems in which detectors are mounted on a mobile platform, typicallycapable of road travel, that traverses a large object to be inspectedsuch as a vehicle or a cargo container. FIG. 11 shows such a system ofthe mobile sort, by way of example, wherein detectors contained withindetector modules 800 and 802, as further described below, are carriedby, or otherwise coupled to, truck 824 which traverses an enclosure tobe inspected during the course of the inspection. The detectors may besensitive both to emission naturally emitted by threat materials as wellas to penetrating radiation that is emitted by a source carried by, orotherwise coupled to, truck 824, after the penetrating radiation hasinteracted with the object under inspection.

Inspection systems that may be used for practice of the presentinvention are of particular utility for the inspection of large cargocontainers such as trucks or sea/air containers in that they employmobile platforms that may be driven past the inspected container duringthe course of the inspection. Such systems are described in U.S. Pat.No. 5,764,683 (Swift et al.), issued Jun. 9, 1998, which is incorporatedherein by reference.

In FIG. 12, cargo container inspection system 820 is shown deployed forinspection of passenger cars 822 and 823. FIG. 13 shows anotherembodiment of the invention that may advantageously be employed for theinspection of passenger vehicles.

With reference to FIGS. 12 and 13, a truck 824, typically 35′ long×8′wide×10′6″ high, houses and supports the x-ray inspection equipment,ancillary support and analysis systems, and a hydraulic slow-speed drivemechanism to provide the scan motion. Truck 824 serves as both theplatform on which the mobile system is transported to its intendedoperating site, and a bi-directional translation stage, otherwisereferred to herein as a “bed,” to produce the relative motion requiredduring a scan. Chopper 826 (shown in FIG. 12) is used, in accordancewith flying spot generation (discussed above in reference to FIG. 1) toscan beam 828 of penetrating radiation recursively in a verticaldirection. Radiation scattered by the contents of the cargo container,shown here as passenger car 823, is detected by x-ray backscatterdetectors 830. Boom 832 allows beam stop 834 to intercept beam 828 as itemerges from the distal side of the scanned cargo container. Beam stop834 is also referred to as a “beam catcher.” In addition oralternatively to beam stop 834, an x-ray transmission detector may bemounted in opposition to beam 828. It is to be understood that thepositions of the source 840 and the transmission detector 34 may bereversed, and that source 840 may be carried on the side of the cargocontainer that is distal to truck 824. It is, furthermore, to beunderstood that the term ‘source’ as used herein and in any appendedclaims, and as designated by numeral 840 in the drawings, refers to theentirety of the apparatus (designated by numeral 12 in FIG. 1) used togenerate beam 828, and may have internal components that include,without limitation, apertures, choppers, collimators, etc.

The described embodiments of the invention are intended to be merelyexemplary and numerous variations and modifications will be apparent tothose skilled in the art. All such variations and modifications areintended to be within the scope of the present invention as defined inthe appended claims.

We claim:
 1. A method for creating an x-ray image of an object anddetecting clandestine nuclear material associated with the object, themethod comprising: a. illuminating the object with penetratingradiation; b. detecting emission, including penetrating radiation,emanating from the object; c. producing an x-ray image of the objectbased on the detected emission; and d. distinguishing between detectedemission due to scattered penetrating radiation with the object anddetected emission due to the clandestine nuclear material.
 2. A methodaccording to claim 1, wherein distinguishing includes distinguishingdetected emission due to fissile material.
 3. A method according toclaim 1, wherein distinguishing includes distinguishing on the basis ofx-rays emitted by the object.
 4. A method according to claim 1, whereindistinguishing includes distinguishing on the basis of at least one ofgamma rays and neutrons emitted by the object.
 5. A method according toclaim 1, wherein distinguishing includes distinguishing detectedemission due to a dirty bomb.
 6. A method according to claim 1, whereinilluminating the object includes illuminating the object intermittently,and distinguishing includes distinguishing based on at least the source-and detected-signal timing.